1. Technical Field
This invention relates to a method in which impurities as donors or acceptors are added to a semiconductor base material to form a region which is different in impurity density from the base material.
2. Prior Art and Difficulties
In order to form a semiconductor region of this type, a thermal diffusion method, an epitaxial growth method, or an ion implantation method has been employed. In each of the conventional methods, it is necessary to subject the semiconductor base material to a heat treatment of 800.degree. to 1250.degree. C. In such a high temperature treatment, crystal defects are formed in the semiconductor base material, and heavy metal elements from the heat treatment over are diffused in the semiconductor base material. This decrease the lifetime of carriers.
In addition, in the case of silicon having a high specific resistance of 10 k.OMEGA.-cm or higher, oxygen contained in the crystal acts as a donor with the result that the specific resistance is decreased. Thus, it is difficult to maintain the characteristics of the base material crystal unchanged.
These difficulties may be eliminated by decreasing the heat treatment temperature. However, if the temperature is merely decreased in the conventional methods, variation in the impurity density and diffusion depth of the semiconductor region is increased, and the reproducibility is lowered. For instance when the thermal diffusion method is employed, the coefficient of diffusion of impurities added to the semiconductor base material is so low that it is substantially impossible to perform thermal diffusion at 800.degree. C. or lower.
Furthermore, it is considerably difficult to form an extremely thin semiconductor region, such as one less than 0.2 .mu.m in depth near the surface of the base material by using the conventional methods. In order to form the extremely thin semiconductor region by the ion implantation method, it is necessary to set the accelerating voltage to 30 kV or to form an oxide film on the semicondutor base material and to implant impurity ions, as dopants, through the oxide film thereinto. However, in the former case, as the accelerating voltage is decreased, it becomes difficult to obtain the ion current and accordingly to obtain a high surface impurity density of 10.sup.21 to 10.sup.22 atoms/cm.sup.3. In the latter case, variation in the thickness of the oxide film affects the surface impurity density and diffusion depth of the extremely thin semiconductor region, and it is therefore impossible to obtain an impurities-added layer having a surface impurity density of 10.sup.20 atoms/cm.sup.3 or more. Thus, none of the methods is practical.
When a semiconductor radiation detecting element is manufactured by using a high purity, high specific resistance silicon element having a specific resistance of 10 k.OMEGA.-cm or higher, the high temperature treatment decreases the carrier life time of the semiconductor base material and lowers the S/N ratio. Moreover, if the surface doping region formed is more than 5 .mu.m in thickness, the region insensitive to radiation is increased in thickness, thus lowering the detection sensitivity.
On the other hand, in order to form an impurities-added region in a noncrystalline semiconductor for use in the industrial field, a variety of methods has been proposed. In the most typical method of injecting, for instance boron elements, a monosilane gas (SiH.sub.4) and a diboron gas (B.sub.2 H.sub.6) are simultaneously supplied into a reaction chamber, and glow discharge is caused therein, to obtain a noncrystalline silicon doped with the boron elements. When the method is employed, a thin layer may be formed; however, it is substantially impossible to add boron impurities thereto to a density of 10.sup.21 atoms/cm.sup.3 or more, and the specific resistance thereof cannot be decreased. Furthermore, when the two gases are supplied into the reaction chamber simultaneously, it is difficult to control the flow rates of the gases. Thus, the method is unsatisfactory in reproducibility.